Sentence a is known as a "cleft" sentence, specifically a "wh- cleft" or a "pseudo-cleft". These are common when you want to shift the focus of the sentence. In the specific example of sentences a and b, I think that most speakers would prefer another form of cleft sentence: "It is my right arm that hurts".

Your sentence c is an example of something completely different. This is simple "fronting", i.e. putting a particular element at the beginning of a sentence to emphasize it. In your example, it would suggest something like "in these specific cases" or "in these and only these cases", especially if the word "these" is stressed in speech.